The treatment of internal combustion engine exhaust to convert noxious components such as hydrocarbons (“HC”), carbon monoxide (“CO”) and nitrogen oxide (“NOx”) to innocuous components (water, carbon dioxide and nitrogen) is, of course, well known in the art. Such conversion is attained by contacting the engine exhaust with one or more catalysts, usually comprising an oxidation catalyst and a reduction catalyst, or a so-called three-way conversion catalyst, which has the capability of substantially simultaneously oxidizing HC and CO to water and carbon dioxide, and reducing NOx to nitrogen. Such catalysts often comprise a platinum group metal such as platinum, platinum plus rhodium or palladium dispersed on a refractory inorganic oxide support such as gamma alumina.
A persistent problem in meeting ever more stringent government regulations concerning the discharge of pollutants from engine exhaust is the fact that catalysts, especially oxidation catalysts, require an elevated temperature, usually above 200 or 250° C., in order to attain reasonably high conversion efficiencies. Therefore, during an initial start or other period of engine operation while the engine is cold, referred to as a “cold operation period”, conversion of pollutants, especially hydrocarbons, is carried out with a low efficiency, if at all. Thus, a very substantial proportion of the total oxidizable pollutants, largely comprising hydrocarbons, discharged to the atmosphere during a given period of operation, is discharged during the cold operation period. In order to ameliorate this problem, the art is aware of the expedient of using, in conjunction with the catalyst, a hydrocarbon trap material, such as certain zeolites, which will adsorb hydrocarbons at a low temperature at which the oxidation catalyst is relatively ineffective, and desorb the hydrocarbons only at a more elevated temperature, at which conversion efficiency of the oxidation catalyst is higher than during the cold operation period.
One difficulty with such prior art expedients is that such zeolite materials tend to begin desorbing hydrocarbons, and thus releasing the hydrocarbon to the catalyst, before the catalyst is hot enough to attain acceptably high conversion efficiencies. That is, the prior art inclusion of zeolites, while improving the situation by adsorbing hydrocarbons for a period, commence the desorption too soon after the cold operation period, thereby releasing the hydrocarbons before the catalyst is sufficiently heated, so that only a limited benefit is attained. It would therefore be desirable to have a composition which adsorbs or otherwise traps and retains the hydrocarbons and does not release the hydrocarbons until release temperatures higher than those heretofore attainable are reached by the catalyst used to oxidize the hydrocarbons.
Commonly assigned U.S. Pat. No. 6,171,556, issued Jan. 9, 2001 teaches a method of treating an engine exhaust gas stream containing hydrocarbons and other pollutants at least during a cold-start period of operation. The invention therein provides that the adsorbent material may comprise a molecular sieve material, for example, a molecular sieve material selected from the group consisting of faujasite, chabazite, silicalite, zeolite X, zeolite Y, ultrastable zeolite Y, offretite, and Beta zeolites. In particular, ion-exchanged Beta zeolites may be used, such as Fe/Beta zeolite, or preferably, H/Beta zeolite. The zeolites, preferably Beta zeolites may have a silica/alumina molar ratio of from at least about 25/1, with useful ranges of from about 25/1 to 1000/1. U.S. Pat. No. 6,171,556 is herein incorporated by reference in its entirety.
Preferred zeolites, as discussed in U.S. Pat. No. 6,171,556 include ZSM, Y and Beta zeolites, with Beta zeolites particularly preferred. Most preferably, the adsorbent material is a zeolite that has been treated to remove Bronsted acid sites from the zeolite and has a relative Bronsted acidity, of less than 1.0, preferably less than 0.5. This can be accomplished by leaching the zeolite with an organic or inorganic acid. The zeolite alternatively or additionally can be treated with steam at from 350 to 900° C. with the steam temperature increasing at from 100 to 600° C. degrees per hour. Steam treatment has been found to reduce the relative Bronsted acidity, and result in an increase in the durability of the zeolite when used in hydrocarbon adsorption applications in exhaust gas streams. It has been found that by using such zeolites, the formation of coke during engine testing has been significantly reduced. The material disclosed in U.S. Pat. No. 6,171,556 has been found to effectively adsorb large hydrocarbon molecules, e.g. ≧C4.
Another difficulty with prior art expedients in which the zeolites are ion-exchanged with cations such as silver, copper or other metals, is the perceived need to segregate catalytic components, such as platinum group metals dispersed on an inorganic oxide support material, from metal ions such as silver, which are ion-exchanged or otherwise dispersed on the zeolite particles. This requires additional manufacturing steps and segregated operations in which impregnation of the oxide supports with catalytic metals such as platinum, or platinum and rhodium, is segregated from operations in which metal cations are ion-exchanged into the zeolites. The ion-exchanged zeolite particles and the separately prepared platinum group metal-impregnated support material particles are then admixed with each other, or disposed on separate portions of a substrate, in order to provide the finished hydrocarbon trap/catalyst material.
In commonly assigned U.S. Pat. No. 6,074,973, issued Jun. 13, 2000, there is disclosed a catalyzed hydrocarbon trap material in which silver and palladium are both dispersed onto zeolite particles and onto refractory metal oxide particles. In one embodiment of the invention, the major portion of the silver is dispersed onto the zeolite particles and the major portion of the palladium is dispersed onto the refractory metal oxide particles, with only a minor portion of the silver dispersed onto the metal oxide particles and only a minor portion of the palladium dispersed onto the zeolite particles. The catalyzed hydrocarbon trap material of the invention is made by a method in which the refractory metal oxide particles and the zeolite particles, together with a soluble palladium component and a soluble silver component dispersed or dissolved together in water, are all combined to effectuate the impregnation of the palladium and silver components into the zeolite and metal oxide particles. The resulting slurry may then be deposited on a suitable substrate, such as a honeycomb-type substrate, and fired at an elevated temperature to provide the catalyzed hydrocarbon trap material. It has been found that Ag-ZSM-5 materials formed by the process described in U.S. Pat. No. 6,074,973 can adsorb small hydrocarbon molecules. The entire content of U.S. Pat. No. 6,074,973 is herein incorporated by reference.
In accordance with U.S. Pat. No. 6,074,973 there is provided a method of making a catalyzed hydrocarbon trap material comprising the following steps. Water, a water-soluble silver component, a water-soluble palladium component, particulate refractory inorganic oxide solids, and particulate zeolite solids having a Si to Al atomic ratio of at least 10 and a pore aperture diameter of at least 4 Angstroms are combined to form an aqueous slurry of the solids. The zeolites may comprise one or more of ZSM-5, Beta, Y, MCM-22 and EU-1 zeolites, e.g., one or more of ZSM-5, Beta and Y zeolites. In any case, the resulting slurry is maintained for a time sufficient to disperse at least some of the palladium component and at least some of the silver component onto each of the inorganic oxide solids and the zeolite solids. Optionally, the loading of the palladium component may be greater on the inorganic oxide solids than on the zeolite solids, and the loading of the silver component may be greater on the zeolite solids than on the inorganic oxide solids. The slurry is then dewatered to provide dewatered solids, and the dewatered solids are heated, e.g., in air at a temperature of at least about 500° C. for at least the time required to decompose the counter ions, i.e., the anions such as nitrate or acetate ions, of the metal salts used in the preparation, and to provide the catalyzed hydrocarbon trap material as a dried mixture of solids.
NGK Insulators' Japanese patent document (Kokai) 8-10566 (1996) was published on Jan. 16, 1996 based on Japanese Patent Application 6-153650, filed on Jul. 5, 1994 and entitled “A Catalyst-Adsorbent For Purification Of Exhaust Gases And An Exhaust Gas Purification Method.” The Abstract of this document, below referred to as “the '650 Application”, discloses a catalyst-adsorbent in which a catalyst material effective for decreasing CO, HC and NOx in internal combustion engine exhaust is combined with an adsorbent material that traps hydrocarbon during cold discharge start-ups. The adsorbent material of the '650 Application is comprised of particles of primarily zeolite, preferably zeolite having a silica-to-alumina ratio of 40 or more; these include ZSM-5 and Beta zeolites. The zeolite adsorbent may optionally have metal ions dispersed therein, the presence of ions of high electro-negativity being said to increase the HC adsorptive capacity. Such ions include silver, palladium, platinum, gold, nickel, copper, zinc, cobalt, iron, manganese, vanadium, titanium, and aluminum. The '650 Application discloses that metal cations may be applied to the zeolite by either ion exchange or immersion methods. The presence in the zeolite of at least one ion of elements of Group IB of the Periodic Table (copper, silver, gold) is said in the '650 Application to manifest a high adsorptive capacity for hydrocarbons even in the presence of water. Copper and silver are stated to be preferable and silver ions, which are specified in some of the examples of the '650 Application, exchanged into the zeolite are said to be particularly desirable for adsorbing HC at higher temperatures. It is stated that the ion content of the zeolite should be greater than 20%, and preferably greater than 40%, relative to the aluminum atoms in the zeolite. Example 95 of the '650 Application shows palladium on ceria-stabilized alumina in a first coating layer and silver- and copper-exchanged ZSM-5 in a second coating layer. Table 6 shows ZSM-5 zeolites exchanged only with silver. The '650 Application discloses that in order to improve low-temperature ignition characteristics to the maximum, it is desirable to form a palladium surface coat layer in which palladium is the only noble metal carried on the particles. The '650 Application further discloses the desirability of forming a first coating on a substrate comprised of the adsorbent zeolite material, over which a second coating comprised of a catalyst material containing only palladium catalyst particles is placed. This is stated to provide excellent durability and low-temperature ignition characteristics of the catalyzed trap material.
Development of a hydrocarbon trap for trapping small hydrocarbon molecules is a challenge. Over the years, many materials have been tested for use as hydrocarbon traps for cold start applications. The materials have to be water-resistant, hydrothermally stable and have the right temperature range for adsorption and desorption of hydrocarbons.